ACR X HAD LTSS FROM ACR Y SYS ERROR.

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I WAS WORKING 460 AT ZME ON THE MORNING OF APR/TUE/92. THE MORNING RUSH HAD JUST STARTED AND I WAS WORKING APPROX 11 ACFT. ACR X WAS INBOUND TO BNA FROM OVER CHA AT FL280 PLT'S DISCRETION TO FL240 LEAVING FL280. THIS ACFT IS REQUIRED BY LOA WITH ZTL TO BE AT FL240 XING THE BOUNDARY. ACR WAS INBOUND TO BNA FROM OVER TYS AT FL230 PLT'S DISCRETION TO FL200. THE ZTL LOA REQUIRES ACFT OVER TYS TO BE AT OR BELOW FL230 DSNDING FL200. MY FIRST PLAN WAS TO DSND ACR X TO FL190 AND GET HIM UNDER ACR Y. I DSNDED ACR X TO FL190 AND TURNED HIM TO A 360 HDG. THEN I DECIDED THIS WASN'T GOING TO WORK SO I STOPPED ACR X AT FL210 ABOVE ACR Y WHO I THOUGHT WAS AT FL200; BUT WAS ACTUALLY AT FL230 WITH PLT'S DISCRETION TO FL200. I THEN DSNDED ACR Y TO 17000 AND A FEW SECONDS LATER TO 14000. APPROX 51 SECONDS LATER; I SAW ACR Y LEAVING FL227 AND REALIZED I HAD A PROBLEM. I STOPPED ACR Y AT FL220 AND TURNED ACR X 30 DEGS L. THE CLOSEST THEY CAME TO EACH OTHER WAS 3.8 AND 900 FT. ACR Y NEVER RPTED LEAVING FL230 AND MADE A COMMENT ABOUT HAVING THE TFC ON TCASII WHEN I STOPPED HIM AT FL220.